This chapter focuses on liquefaction of solids, primarily coal and biomass sources such as lignin. In a two-step process the molecular structure is broken into fragments and hydrogenated, increasing the hydrogen-to-carbon ratio and preventing retrogressive reactions. Solvents solubilize smaller molecular weight components and promote hydrogenation; catalyst improves hydrogenation. The use of high-carbon-based feeds to produce alternative fuels and value-added chemicals is discussed. Basic conversion chemistry, chemical structures of coal and lignin biomass are discussed, followed by process technologies for direct liquefaction, liquid quality, process control and modeling, advantages and limitations, and future trends. Direct liquefaction processes can overlap with pyrolysis; low-temperature pyrolysis is included because of the liquids generated.
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